Abstract : The cis and trans barriers to internal rotation in hydrogen peroxide have been accurately determined within the Hartree-Fock approximation using a (4s3pld/2slp) basis set. The calculated trans barrier (384/cm) differs from experiment (386/cm) by only 2/cm. The cis barrier is not well determined by experiment and the calculated value (2921/cm) may well be the most accurate value reported to date. The effects of geometry optimization and polarization functions were also studies. The calculations clearly illustrate the importance of determining the Hartree-Fock geometry for a suitable molecular configuration. With the exception of the OOH bond angle, optimization of the geometrical parameters as the molecule rotates was found to be of little importance; the increase of 6 degrees in <OOH upon rotation from the cis to the trans configuration lowers the cis barrier by 653/cm. It was found that the inclusion of oxygen 3d and hydrogen 2p functions was essential for describing the trans barrier, without which the barrier height was just 54/cm. In addition, polarization functions lowered the cis barrier by 675/cm.